It takes talent to write about someone else’s interesting life in a book that is interesting itself. The biographer would have to be, say, an author, scholar and translator. Such a person could write a biography about an intellectual polyglot, polymath, “journalist, novelist, essayist, autobiographer, and writer of scientific speculations” — and Casanova.

Michael Scammell is such a biographer. His new book on Arthur Koestler (1905-1983) took two decades to complete. He has flushed out details that Koestler himself left out — in some half dozen autobiographies. He shares Koestler’s long, convoluted life in a thoroughly enjoyable read. The British-born Mr. Scammell comes with top credentials…